According to Dutch site PS4Insider the way you pay for your PlayStation Plus membership may be changing soon. In an effort to become more transparent as of October 10th 2013 Sony will reportedly shift the payment schedule into calender months, with the renewal of services falling on the same day month in, month out.
This alteration to the method will remove the confusion and unfortunate arithmetic that was required to estimate renewal dates. Put simply; PlayStation Plus will no longer be renewed every 90 days, but every 3 months on the same day as it was activated.
We here at The Vita Lounge are all for making things easier and we will always get behind doing as little math as possible.